within the last decade, the use of fixation plates for the treatment of spinal disorders or for fusion of vertebrae has grown considerably. While early procedures using fixation plates were generally restricted to long bones and lower lumbar levels of the spine, such fixation plates have recently found applications in other bone instrumentation such as in instrumentation of the cervical spine. Successful spinal instrumentation in this region is particularly difficult given the problems of safely accessing the instrumentation site.
One typical example of a bone fixation plate is provided with a plurality of bores therethrough. A corresponding plurality of fastener members, typically bone screws having a headed portion and an elongated threaded shaft extending therefrom, are provided to secure the plate to a bone, or bones, to be fixated. The bone screws are advanced into the corresponding plate bores and then into the bone itself. The screws are then firmly tightened to thereby secure the plate to the bone.
A common problem associated with the use of such fixation plates, regardless of their location, is the tendency of the bone screws to "back out" of the underlying bone under the stress of bodily movement. This problem is particularly prevalent in areas of high stress such as, for example, the spine. Given the delicate nature of the spine, any fixation plate movement post-operatively may not only frustrate the surgical goals, but may also raise genuine patient safety concerns.
Designers of such bone fixation systems have employed various techniques in an attempt to overcome the foregoing problem. For example, U.S. Pat. No. 5,364,399 to Lowery et al., assigned to the assignee of the present invention, discloses an anterior cervical plating system incorporating a locking screw which engages the heads of bone screws to secure the cervical plate to the vertebra. The locking screw, positioned above the bone screws, provides a rigid fixation of the screws to the plate wherein the heads of the bone screws are either flush with, or recessed below, the upper surface of the plate.
As another example, U.S. Pat. No. 5,275,601 to Gogolewski et al. discloses a self-locking bone fixation system wherein the heads of the bone screws are frustoconical in shape and have a directionally corrugated outer surface. Each bore in the fixation plate has a complementarily corrugated inner surface and is similarly frusto-conical in shape. As the screws are advanced through the corrugated bores and into the underlying bone, the direction of corrugation permits the heads to be received within the corresponding bores, while inhibiting rotation of the screws in an opposite direction.
As a further example, U.S. Pat. No. 5,269,784 to Mast discloses a threaded screw nut for use with a bone fixation system wherein the screw nut is partially insertable into a bore of the fixation plate from the underside thereof, and threadingly engages a portion of the bone screw to thereby secure the bone screw to the fixation plate.
As a final example, U.S. Pat. No. 4,484,570 to Sutter et al. discloses a bone fixation system wherein the heads of the bone screws are hollow and expandable. After the fixation plate is secured to the underlying bone via the hollow head bone screws, set screws are advanced into the hollow heads to expand the heads and thereby secure the heads to the fixation plates.
All of the foregoing prior art systems suffer from several undesirable drawbacks. First, the addition of intricately machined componentry makes most of these systems expensive and difficult to manufacture. Second, since some of these prior art systems rely on a threaded connection to maintain the bone screws in a secure position, such locking systems are still subject to the problem of screw back out and may therefore be unreliable. Finally, most of the foregoing systems, by their nature, result in an increased profile or bulk which, in many surgical applications, is undesirable. What is therefore needed is a bone fixation system including a mechanism for preventing screw backout without increasing the profile or bulk of the fixation system. Such a system should ideally be easy and inexpensive to manufacture.